1. Field of the Invention
The present invention relates to a surface mountable device, and more particularly, to a surface mountable device with a three-dimensional structure.
2. Description of Related Art
FIG. 1 depicts a conventional surface mountable device 10, comprising an upper electrode 11, a lower electrode 12 and a current-sensitive element 13. Nowadays, the current-sensitive element 13 is usually formed of a conductive material having Positive Temperature Coefficient (PTC). The resistance of the PTC conductive material can be kept extremely low at normal operation due to its low sensitivity to temperature variance so that the circuit can operate normally. However, if the over-current or over-temperature effect occurs, the resistance will immediately be increased thousands of times to a higher resistance state (e.g. above 104 ohm.) Therefore, the over current will be reversely eliminated and the objective to protect the circuit device can be achieved.
Generally, the normal resistance value of the current-sensitive element 13 by follows the conventional formula:       R    =                  ρ        xc3x97        l            A        ,
in which xcfx81 is the conductive coefficient of the PTC conductive composition, l is the length and A is the area of the current-sensitive element 13. Since the size of the printed circuit board of the portable electronic product decreases more and more the footprint of the surface mountable device mounted on the circuit board also needs to be reduced comparatively. Thus, according to the above formula, the normal resistance of the surface mountable device will be increased.
In addition, since the conventional surface mountable device is a planar structure, it will occupy a lot of surface area of the circuit board when the convention surface mountable device is mounted. To solve this space limitation problem, Raychem Corp. discloses a vertical surface mountable device 20 (Model TS250), comprising a conventional surface mountable device 10, a covering 25, an insulating material 24, a first conductive element 26 and a second conductive element 27, as shown in FIG. 2. The covering 25 is used to cover the conventional surface mountable device 10 in which the surface mountable device 10 is vertically placed in the covering 25. The first conductive element 26 and the second conductive element 27 are disposed on the bottom surface of the vertical surface mountable device 20 and contacts the first electrode 11 and the second electrode 12 of the conventional surface mountable device 10 respectively to be mounted on the circuit board (not shown.) Further, more an insulating material 24 is filled into the vacant space between the covering 25 and the two electrodes 11, 12. Although the mounted footprint on the surface mountable device 20 by the surface mountable device can be reduced by the above structure, the current leakage occurs more easily because the first conductive element 26 and the second conductive element 27 are disposed too closely. Because the structure is covered with the covering 25, filling the insulating material 24 is difficult and dissipating the heat is also difficult. Moreover, the voltage endurance of the above structure has an upper limit (about 60V) which is not suitable for the products requiring high voltage endurance.
A major objective of the present invention is to provide a surface mountable device, wherein the shape of the device can be varied according to the available space of the circuit board so that the requirement of a light, thin and small circuit board can be met.
A second objective of the present invention is to provide a surface mountable device, wherein the shape of the device is a three-dimensional structure, different from the two-dimensional planar structure of the conventional surface mountable device, so as to increase the area of the PTC conductive material and reduce the normal resistance and increase the voltage endurance up to above 200V. In other words, the surface mountable device of the present invention can be applied to products requiring high voltage endurance.
A third objective of the present invention is to provide a surface mountable device, wherein the mounted direction can be varied according to the requirement. Thus, the surface space of the circuit board can be used more efficiently.
A fourth objective of the present invention is to provide a surface mountable device, which can be directly mounted onto the circuit board and be easily filled with insulating material; therefore short circuit will not occur and the heat inside the device is dissipated easily.
In order to achieve the above objectives and to avoid the disadvantages of the prior art, the present invention discloses a surface mountable device comprising a current-sensitive element and two electrodes in which the current-sensitive element is composed of a PTC conductive composite material having a positive temperature coefficient, comprising at least one polymer and a conductive filler. The present invention is characterized in that the current-sensitive element is a three-dimensional bent structure so that the shape, length and height of the element can be varied according to the requirement of mounting space and resistance and thus the surface space of the circuit board can be used more efficiently. Moreover, the area of the current-sensitive element of the present invention is larger than that of the conventional surface mountable device. Consequently, the normal resistance of the surface mountable device of the present invention is smaller than that of the conventional surface mountable device and the voltage endurance of the surface mountable device of the present invention is increased accordingly.
The foregoing and other objectives and advantages of the invention, as well as the manner in which the same are accomplished, will become clearer based on the following detailed description taken in conjunction with the accompanying drawings.